mac80211: improve rate handling

Some code currently assumes that there's a valid
rate pointer even in the HT case, but there can't
be. To reduce reliance on that, remove the rate
pointer from the RX data struct and pass it where
it's needed.

Also, for now, in radiotap announce HT frames as
having a DYN channel type, and remove their rate
from cooked monitor radiotap completely (it isn't
present in the regular monitor radiotap either.)

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>